Wafer processing method

ABSTRACT

A wafer processing method including the step of removing a ringlike reinforcing portion formed along the outer circumference of a wafer on the back side thereof. The ringlike reinforcing portion is ground by a grinding stone in such a manner that the locus of the grinding stone rotating intersects the ringlike reinforcing portion as viewed in plan. The grinding of the ringlike reinforcing portion is ended when the ground surface of the ringlike reinforcing portion becomes higher by 20 to 1 μm than the upper surface of a metal film deposited on the back side of a device area of the wafer. It is unnecessary to accurately align the grinding stone to the ringlike reinforcing portion on the upper side thereof, so that the position control can be easily performed. Further, the grinding of the ringlike reinforcing portion is ended when the difference in height between the ground surface of the ringlike reinforcing portion and the upper surface of the metal film becomes 20 to 1 μm, so that there is no possibility that the metal film may be damaged.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer processing method by which a thin wafer can be easily handled.

2. Description of the Related Art

A plurality of devices such as ICs and LSIs are formed on the front side of a wafer, and the wafer is separated into the individual devices by using a dicing unit or the like. These devices are widely used in various electronic equipments. For the purposes of reduction in size and weight of electronic equipment, the back side of the wafer before separated into the individual devices is ground to obtain a thickness of 20 to 100 μm, for example. However, the wafer having such a small thickness obtained by grinding is reduced in stiffness, so that it is difficult to handle and transfer in the subsequent steps. For example, it is difficult to deposit a metal film such as a gold, silver, or titanium film having a thickness of tens of nanometers on the back side of the wafer having a small thickness obtained by grinding.

To solve this problem, the present applicant has already proposed a wafer processing method including the steps of grinding the back side of a wafer in a device area where devices are formed to obtain a predetermined thickness and to form a ringlike reinforcing portion along the outer circumference of the wafer, next depositing a metal film on the back side of the wafer in the condition where the stiffness of the wafer reduced in thickness is increased by the ringlike reinforcing portion to ease the handling and transfer of the wafer, next removing the ringlike reinforcing portion, and finally dicing the wafer to separate it into the individual devices (see Japanese Patent Laid-open No. 2007-19379). In this wafer processing method described in Japanese Patent Laid-open No. 2007-19379, however, a grinding stone for grinding the ringlike reinforcing portion formed on the back side of the wafer must be accurately aligned to the ringlike reinforcing portion, so as not to grind the metal film deposited on the back side of the device area of the wafer. Accordingly, the position control for the grinding stone and the wafer becomes troublesome.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a wafer processing method including the steps of grinding the back side of the device area of a wafer to form a ringlike reinforcing portion along the outer circumference of the wafer, next depositing a metal film on the back side of the wafer, and next removing the ringlike reinforcing portion, wherein the ringlike reinforcing portion can be easily removed without damaging the metal film.

In accordance with an aspect of the present invention, there is provided a wafer processing method including a ringlike reinforcing portion forming step for holding a wafer on a chuck table of a first grinding unit in the condition where the front side of the wafer is placed on the chuck table, the front side of the wafer being composed of a device area where a plurality of devices are formed so as to be partitioned by a plurality of streets and an outer circumferential marginal area surrounding the device area, and for grinding the back side of the device area of the wafer to form a recess and a ringlike reinforcing portion around the recess; a metal film deposition step for depositing a metal film on the back side of the wafer after performing the ringlike reinforcing portion forming step; and a ringlike reinforcing portion removing step for removing the ringlike reinforcing portion after performing the metal film deposition step. The ringlike reinforcing portion removing step uses a second grinding unit including a rotatable chuck table having a holding surface for holding the wafer, grinding means having a rotatable grinding wheel and a ringlike grinding stone fixed to the grinding wheel for grinding the wafer held on the chuck table, and feeding means for feeding the grinding means in a direction perpendicular to the holding surface of the chuck table. The ringlike reinforcing portion is ground by the grinding stone in such a manner that the locus of the grinding stone rotating intersects the ringlike reinforcing portion as viewed in plan as rotating the chuck table on which the front side of the wafer is held and feeding the grinding stone by operating the feeding means during rotation of the grinding wheel; the grinding of the ringlike reinforcing portion being ended when the ground surface of the ringlike reinforcing portion becomes higher by 20 to 1 μm than the upper surface of the metal film deposited on the back side of the device area.

Preferably, the wafer processing method further includes a separating step for separating the wafer into the individual devices along the streets in the condition where a dicing tape is attached to the back side of the wafer and the wafer is supported through the dicing tape to a dicing frame, after performing the ringlike reinforcing portion removing step. In this case, the dicing tape preferably has a thickness of 80 to 100 μm.

According to the present invention, in the ringlike reinforcing portion removing step, the ringlike reinforcing portion is ground by the grinding stone in such a manner that the locus of the grinding stone rotating intersects the ringlike reinforcing portion as viewed in plan. Accordingly, it is unnecessary to accurately align the grinding stone to the ringlike reinforcing portion on the upper side thereof, so that the position control can be easily performed. Further, the grinding of the ringlike reinforcing portion is ended when the difference in height between the ground surface of the ringlike reinforcing portion and the upper surface of the metal film formed on the back side of the device area becomes 20 to 1 μm. Accordingly, there is no possibility that the grinding stone may come into contact with the metal film, thereby preventing the damage to the metal film.

In the separating step after the ringlike reinforcing portion removing step, the back side (the ground surface) of the ringlike reinforcing portion projecting by 20 to 1 μm from the upper surface (the exposed surface) of the metal film deposited on the back side of the device area is attached to the dicing tape. Such a height difference is sufficiently smaller than the thickness of the dicing tape, and the dicing tape is formed of a soft material. Accordingly, this height difference can be absorbed by the soft dicing tape, so that there is no trouble in cutting the wafer.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a wafer and a protective member;

FIG. 2 is a perspective view showing the wafer in the condition where the protective member is attached to the front side of the wafer;

FIG. 3 is a perspective view showing a ringlike reinforcing portion forming step;

FIG. 4 is a perspective view showing the wafer after the ringlike reinforcing portion forming step;

FIG. 5 is a sectional view of the wafer shown in FIG. 4;

FIG. 6 is a schematic sectional view showing a low-pressure film deposition unit usable in a metal film deposition step;

FIG. 7 is a sectional view of the wafer after the metal film deposition step;

FIG. 8 is a perspective view showing a grinding unit usable in a ringlike reinforcing portion removing step;

FIG. 9 is a perspective view showing the ringlike reinforcing portion removing step;

FIG. 10 is a sectional view of the wafer after the ringlike reinforcing portion removing step;

FIG. 11 is a perspective view showing the step of attaching the wafer shown in FIG. 10 to a dicing tape and removing the protective member from the wafer;

FIG. 12 is a perspective view showing a cutting unit usable in a separating step; and

FIG. 13 is an enlarged sectional view showing the attached condition of the wafer to the dicing tape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, the front side Wa of a wafer W is composed of a device area W1 where a plurality of devices D are formed and an outer circumferential marginal area W2 surrounding the device area W1. The plural devices D are separated by a plurality of crossing streets S in the device area W1. The outer circumferential portion of the wafer W is formed with a notch N indicating the crystal orientation.

A protective member 1 such as a tape is attached to the front side Wa of the wafer W. As shown in FIG. 2, the wafer W is turned upside down in such a manner that the back side Wb of the wafer W is directed upward. As shown in FIG. 3, the back side Wb of the wafer W is ground by using a grinding unit 2 in the condition where the back side Wb is exposed. The grinding unit 2 includes a rotatable chuck table 20 for holding the wafer W and grinding means 21 for grinding the wafer W held on the chuck table 20. The grinding means 21 includes a spindle 22 rotatable and vertically movable, a grinding wheel 23 mounted on the lower end of the spindle 22 so as to be rotatable by the rotation of the spindle 22, and a grinding stone 24 fixed to the lower surface of the grinding wheel 23.

The wafer W is held on the chuck table 20 in the condition where the protective member 1 attached to the front side Wa of the wafer W is placed on the chuck table 20 and the back side Wb of the wafer W is opposed to the grinding stone 24. When the chuck table 20 is rotated to rotate the wafer W, the grinding stone 24 rotating with the rotation of the spindle 22 is lowered to come into contact with the back side Wb of the wafer W. More specifically, the grinding stone 24 comes into contact with the back side Wb at its inner area corresponding to the device area W1 of the front side Wa (see FIG. 1), i.e., the back side of the device area W1, so as not to grind the remaining outer circumferential area of the back side Wb. Accordingly, as shown in FIGS. 4 and 5, the inner area of the back side Wb is ground to form a recess W3, and the remaining outer circumferential area of the back side Wb is not ground to make difference in level on the recess W3. In other words, the ringlike reinforcing portion W4 is formed on the back side of the outer circumferential marginal area W2 (ringlike reinforcing portion forming step). Preferably, the ringlike reinforcing portion W4 has a thickness of hundreds of micrometers. On the other hand, the thickness of the device area W1 can be reduced to 20 to 100 μm, for example.

After performing the ringlike reinforcing portion forming step, a metal film such as a gold, silver, or titanium film is deposited on the back side Wb of the wafer W (metal film deposition step). Prior to performing the metal film deposition step, the wafer W with the protective member 1 is removed from the chuck table 20 of the grinding unit 2 shown in FIG. 3. As compared with the case of removing a wafer having a fully ground back side from a chuck table, the wafer W having the ringlike reinforcing portion W4 according to this preferred embodiment can be easily removed without the possibility of damage.

In performing the metal film deposition step, a low-pressure film deposition unit 3 shown in FIG. 6 may be used. The low-pressure film deposition unit 3 has a chamber 31 and a holding member 32 provided in the chamber 31 for electrostatically holding the wafer W. A sputter source 34 formed of metal is provided in the chamber 31 at an upper position opposed to the holding member 32 so as to be supported by an exciting member 33. A high-frequency power supply 35 is connected to the sputter source 34. A gas inlet 36 for introducing a sputter gas into the chamber 31 is formed through one side wall portion of the chamber 31, and an evacuation hole 37 communicating with a low-pressure source is formed through another side wall portion of the chamber 31.

The wafer W is electrostatically held on the holding member 32 in the condition where the protective member 1 attached to the front side Wa of the wafer W is placed on the holding member 32, so that the back side Wb of the wafer W is opposed to the sputter source 34. A high-frequency power having a frequency of about 40 kHz is applied from the high-frequency power supply 35 to the sputter source 34 magnetized by the exciting member 33, and the chamber 31 is evacuated from the evacuation hole 37 to a low pressure of about 10⁻² to 10⁻⁴ Pa. Thereafter, an argon gas is introduced from the gas inlet 36 into the chamber 31 to generate a plasma. As a result, argon atoms in the plasma collide with the sputter source 34 to eject the particles from the surface of the sputter source 34. The ejected particles are deposited to the back side Wb of the wafer W to form a metal film 4 as shown in FIG. 7. The metal film 4 has a thickness of about 30 to 60 nm, for example. In the case that the ringlike reinforcing portion W4 is masked in the metal film deposition step, the metal film 4 is formed on only the bottom surface of the recess W3. The metal film deposition step is performed in the condition where the back side of the device area W1 of the wafer W has been ground to reduce the thickness of the wafer W in the device area W1. However, since the wafer W has the ringlike reinforcing portion W4, the wafer W is easy to handle in the metal film deposition step. Any other film deposition methods such as evaporation and CVD may be used in the metal film deposition step.

After performing the metal film deposition step, the ringlike reinforcing portion W4 is removed (ringlike reinforcing portion removing step). In this preferred embodiment, the ringlike reinforcing portion removing step is performed by using a grinding unit 5 shown in FIG. 8. The grinding unit 5 includes a chuck table 6 having a holding surface 60 for holding the wafer W, the chuck table 6 being rotatable and movable in a horizontal direction, grinding means 7 for grinding the wafer W held on the holding surface 60 of the chuck table 6, and feeding means 8 for feeding the grinding means 7 in a direction perpendicular to the holding surface 60.

The grinding means 7 includes a spindle 70 having a vertical axis, a spindle housing 71 for rotatably supporting the spindle 70, a wheel mount 72 formed at the lower end of the spindle 70, a grinding wheel 73 fixed to the wheel mount 72, a ringlike grinding stone 74 fixed to the lower surface of the grinding wheel 73, and a motor 75 for driving the spindle 70.

The feeding means 8 includes a ball screw 80 extending in the vertical direction, a pulse motor 81 connected to one end of the ball screw 80, a pair of guide rails 82 extending parallel to the ball screw 80, a lift plate 83 having a nut portion (not shown) for threadedly engaging the ball screw 80 and a pair of vertically extending side portions for slidably engaging the guide rails 82, respectively, and a supporting portion 84 connected to the lift plate 83 for supporting the spindle housing 71. When the pulse motor 81 is operated to rotationally drive the ball screw 80, the lift plate 83 is vertically moved by the engagement of the nut portion and the ball screw 80 as being guided by the guide rails 82. Accordingly, the supporting portion 84 and the grinding means 7 are vertically moved together. A pulse signal is supplied from a control section (not shown) to the pulse motor 81 to thereby precisely control the vertical position of the grinding stone 74 in micrometers.

The wafer W having the ringlike reinforcing portion W4 and the metal film 4 as shown in FIG. 7 is held on the holding surface 60 of the chuck table 6 in the condition where the protective member 1 attached to the front side Wa of the wafer W is placed on the holding surface 60. In this condition, the chuck table 6 is rotated to thereby rotate the wafer W. At the same time, the motor 75 is driven to rotate the grinding wheel 73, and the grinding means 7 is lowered by the feed operation of the feeding means 8. As shown in FIG. 9, the ringlike reinforcing portion W4 of the wafer W is ground by the grinding stone 74 in such a manner that the locus of the grinding stone 74 rotating intersects the ringlike reinforcing portion W4 as viewed in plan. As shown in FIG. 10, the grinding of the ringlike reinforcing portion W4 is ended when the ground surface W4 a of the ringlike reinforcing portion W4 becomes higher by 20 to 1 μm than the upper surface 4 a of the metal film 4 (ringlike reinforcing portion removing step). In other words, the ringlike reinforcing portion W4 is ground until the height of the reinforcing portion W4 from the upper surface 4 a of the metal film 4 is reduced to 20 to 1 μm.

Thus, the feed operation of the feeding means 8 is stopped at the time the lower surface of the grinding stone 74 reaches a vertical position raised by 20 to 1 μm from the upper surface 4 a of the metal film 4. Accordingly, there is no possibility that the grinding stone 74 may come into contact with the metal film 4, thereby preventing the damage to the metal film 4. Further, the ringlike reinforcing portion W4 of the rotating wafer W is ground by the rotating grinding stone 74 in such a manner that the locus of the grinding stone 74 intersects the ringlike reinforcing portion W4 as viewed in plan as mentioned above. Accordingly, it is unnecessary to align the grinding stone 74 to the ringlike reinforcing portion W4 on the upper side thereof, and the position control of the chuck table 6 in the horizontal direction can also be easily performed.

After performing the ringlike reinforcing portion removing step, the wafer W is attached to a dicing tape T in the condition where the protective member 1 is exposed as shown in FIG. 11. Further, the outer circumferential portion of the dicing tape T is attached to a ringlike dicing frame F as shown in FIG. 11. Accordingly, the wafer W is supported through the dicing tape T to the dicing frame F. Further, the protective member 1 is removed from the front side W1 of the wafer W as shown in FIG. 11. The dicing tape T has a thickness of about 80 to 100 μm and it is formed of a soft material such as polyolefin. The wafer W thus supported through the dicing tape T to the dicing frame F is transferred to a chuck table 90 of a cutting unit 9 shown in FIG. 12 and is held on the chuck table 90.

The cutting unit 9 includes the rotatable chuck table 90 for holding the wafer W and cutting means 91 for cutting the wafer W held on the chuck table 90. The cutting means 91 includes a housing 910, a spindle 911 rotatably supported to the housing 910, and a cutting blade 912 mounted on the front end of the spindle 911. The chuck table 90 is movable in the X direction by work feeding means 92. The cutting means 91 is movable in the Y direction by indexing means 93. The cutting means 91 is also movable in the Z direction by tool feeding means 94.

The dicing tape T attached to the back side Wb of the wafer W is held on the chuck table 90. As mentioned above, the difference in height between the ground surface W4 a of the ringlike reinforcing portion W4 and the upper surface 4 a (the lower surface as viewed in FIG. 13) of the metal film 4 is in the range of 20 to 1 μm. This difference in height can be absorbed by the soft dicing tape T having a thickness of 80 to 100 μm, so that the front side Wa (the upper surface as viewed in FIG. 13) of the wafer W can be made flat in the condition where the wafer W is held on the chuck table 90, and there is no trouble in cutting the wafer W.

After holding the wafer W on the chuck table 90, the chuck table 90 is moved in the X direction to a position directly below the cutting blade 912. The cutting blade 912 rotating at a high speed is lowered to cut the wafer W along a predetermined or detected street extending in the X direction as moving the chuck table 90 in the +X direction. Thus, the predetermined street formed on the front side Wa of the wafer W is cut by the cutting blade 912. After cutting this predetermined street, the cutting means 91 is indexed in the Y direction by the amount corresponding to the space between the adjacent streets, and the next street adjacent to the above predetermined street is similarly cut by the cutting blade 912. This cutting operation is repeated for all of the streets extending in the X direction. Thereafter, the chuck table 90 is rotated 90° to similarly cut the remaining streets extending to the above cut streets. Thus, all of th perpendicular e crossing streets formed on the front side Wa of the wafer W are cut to obtain the individual separated devices D.

The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention. 

1. A wafer processing method comprising: a ringlike reinforcing portion forming step for holding a wafer on a chuck table of a first grinding unit in the condition where the front side of said wafer is placed on said chuck table, the front side of said wafer being composed of a device area where a plurality of devices are formed so as to be partitioned by a plurality of streets and an outer circumferential marginal area surrounding said device area, and for grinding the back side of said device area of said wafer to form a recess and a ringlike reinforcing portion around said recess; a metal film deposition step for depositing a metal film on the back side of said wafer after performing said ringlike reinforcing portion forming step; and a ringlike reinforcing portion removing step for removing said ringlike reinforcing portion after performing said metal film deposition step; said ringlike reinforcing portion removing step using a second grinding unit having a rotatable chuck table having a holding surface for holding said wafer, grinding means having a rotatable grinding wheel and a ringlike grinding stone fixed to said grinding wheel for grinding said wafer held on said chuck table, and feeding means for feeding said grinding means in a direction perpendicular to said holding surface of said chuck table; said ringlike reinforcing portion being ground by said grinding stone in such a manner that the locus of said grinding stone rotating intersects said ringlike reinforcing portion as viewed in plan as rotating said chuck table on which the front side of said wafer is held and feeding said grinding stone by operating said feeding means during rotation of said grinding wheel; the grinding of said ringlike reinforcing portion being ended when the ground surface of said ringlike reinforcing portion becomes higher by 20 to 1 μm than the upper surface of said metal film deposited on the back side of said device area.
 2. The wafer processing method according to claim 1, further comprising a separating step for separating said wafer into said individual devices along said streets in the condition where a dicing tape is attached to the back side of said wafer and said wafer is supported through said dicing tape to a dicing frame, after performing said ringlike reinforcing portion removing step.
 3. The wafer processing method according to claim 2, wherein said dicing tape has a thickness of 80 to 100 μm. 